Mechanics of Fluids, Civil, and Mechanical Engineering - Assignment Example

Name: Title: Date: Part B. (LO 1) Heat transfer The underside of a smoky layer 10m x 8m is radiating like a flat, isotropic plate at 475C to the floor of a compartment 1.20m below. The mean emissivity is 0.35 and the floor is homogenous/flat plate at 48C. What is the rate of heat transfer from the smoke to the floor? Where and Useful figures & formulae: Part C. (LO 1) fluid flows a) Critically discuss the “ventilation parameter” and how it relates (if at all) to the mass flow at the doorway of a fire-compartment, to the neutral plane, and to the thermal discontinuity plane. (10 marks) Ventilated-controlled fires in a compartment can be studied using a rectangular opening located at the central location of a wall. The main factors that affect the burning behaviour in a compartment include compartment shape and size, the opening condition, the type of burning material, the area of the fuel bed. The effects of the shape and size of the ventilation opening is expressed by ventilation parameter, A, where A is the area of the ventilation opening, and H is its height (Hurley, 2016). The fire plume entrains the gases from the lower cold later to the upper hot layer at a rate me. Under a steady stead condition, the mass flow rate through a vent from the hot layer, mg, is equivalent to the fuel flow, mf, plus the rate of air flow in the compartment, ma. The mass flows in and out of the compartment are related by: is the rate of burning of a mass of fuel (kg/s), mg is the mass flow rate of the gas through an opening and ma is the air mass flow rate into a window or a door. It has been observed that fuel in a compartment with a small opening seems to burn at a rate approximately stoichiometric. An experiment on wood fires led to the following equation. (Hurley, 2016) The hot gases in the upper part of the compartment create a pressure difference in respect to the hydrostatic pressure across the opening. This relationship causes the flow. The gas in the compartment is stationary and the flow rate is calculated using Bernoulli’s equation with an associated orifice coefficient. In addition, the gas flow in and out of the compartment occurs through entrainment of the fire plume. The figure shows the room geometry. The figure shows the interrelation between the difference in the door flow, temperature and pressure difference. The interaction between the temperature and thermal discontinuity in the height of two meters from the floor level is assumed and it usually leads to an integral equation. The mass flow velocity varies with respect to the height of the neutral plane, and its relationship produce an integrated function as follows. Where min is the mass in flow, is the co-efficiency of the orifice constriction (usually ~ 0.7), is the ambient air density (kg/m2), Win is the width of the opening If Vin is substituted then, By replacing dy, the equation becomes. Where N is the height to the neutral plane and H0 is the height to the top of the vent. b) The governing equations of a fluid dynamics model are shown below. Explain the physical sense of each term. (10 marks) Continuity equation The equation is a partial differential equation that represents a continuous velocity. The term I describes the change in density with time; it also represent the shrinkage-flow in heat transfer solidification. The second term represents the influence of the velocity. Momentum equation Conservation momentum equation from mean flow in a turbulent field is shown by the equation below. This equation comes from application of the Newton’s 2nd law to an element of fluid. The left side of the equation (I and II) expresses the rate of change of momentum of one unit volume of the fluid. The term I represent the familiar temporary acceleration, and the term II represents the convective acceleration and will accounts acceleration due to the presents of an obstacles or when there is a steady flow. The right side of the equation represents the forces causing the acceleration. The term III accounts for the influence of the mean pressure gradients. The tern V represents the influence of body forces like gravity that act on the fluid mass. The term IV represents influence of the turbulent stress on the mean motion or the difference in turbulent momentum flux. This term needs turbulence modelling in order to predict the effects of turbulence. The term that include represents the influence of shear stress (Currie, 2012). Mass equation Where with etc The left hand side of the equation represents the change in quantity as seen by an observer who is watching a given mass of fluid. The term I represents the fact that at any point in space, the properties of fluid may change with time. The term II represent the fact that in a flow field that is independent of time, and the properties of the fluid is dependent on the spatial coordinate, there is a change due to the change in position of the fluid element with time and assumes different values as it flows. The term III represents the influence by mean pressure gradients. Energy equation Where with ; The left hand side of the equation represents the rate of change of internal energy. The term I represent the temporary change and the term II represent the local convective change due to the flow of fluid between two areas. The right hand side represents the reason for change in the internal energy. The term III represents the influence by mean pressure gradients. The term IV represents the influence due to the conversion of mechanical energy into thermal energy as a result of the action of the stresses on the surface. The term V represents the rate at which heat is increased due to convection from outside (Currie, 2012). Part D. (LO 4) Fire supression (a) Analyse three conditions essential for combustion and fire (the fire triangle). Specify three associated methods of fire-fighting and relate these methods with the action of water, foam, or neutral gas. (5 marks) The combustion and fire occurs in the presence of three conditions which interact together. The conditions are: - A source of heat (thermal energy or activation energy) - An oxidizer (oxygen) - Fuel (combustible material) The conditions are normally represented in fire triangle as shown below. Each side of the triangle represents one condition and the triangle symbolizes the interaction among the three conditions, which produce fire (Siegel, & Saukko, 2013). Fire is stopped by removing one side of the triangle. The principle of fire extinguishment is to inhibit the interaction of one condition with the other two. The basic methods that are associated with the fire triangle include: Cooling the burning material Excluding oxygen from fire Removing fuel from fire (Siegel, & Saukko, 2013) (b) Critically review different mechanisms of fire extinguishment (cooling of flame, reduction of fuel and/or oxygen, and interference with combustion reactions). (10 marks) Cooling of flame The most common method for extinguishing fire is cooling the burning material using water. Water absorbs heat from fire as it changes into steam. It serves as a heat sink by reducing the temperature of the fuel. The gaseous fuel can also be cooled by use of water vapour as it has a relatively higher heat capacity compared to most fuel. If the fuel composed of a hydrocarbon, water will spread the flame as they are immiscible. Reduction of fuel and/or oxygen Another method is application of foam to a petroleum fire. It not only cools the surface of the fuel, but it also creates a blanket over it which separates the oxygen from the fuel, and prevents any gasified fuel from rekindling the flame. Interference with combustion reactions The method involves blocking the fuel by closing the supply pipes. The supply of oxygen can also be stopped by closing the openings to the burning fuel. This technique causes depletion of oxygen at the atmosphere surrounding the flame and the flame is extinguished (International Association of Fire Chiefs, 2004). (c) Review fire protection using water. Analyse the reason for Halon phase-out. (5 marks) The use of water to distinguish fire has been common. Water mist is used to suppress fire by generating and distributing water mist within the protected space. Small droplets of mists extinguish the flame through the combined effects of cooling and dilution of oxygen through the steam generated after evaporating water. The techniques used to generate water mists are: High pressure single fluid atomisation Dual fluid atomization Hot water steam generation Low pressure single fluid atomisation The water mists droplets used to extinguish fire has diameter of less than 200µm. This technique uses mechanisms such as: Steam formation resulting in oxygen dilution Cooling of the gas phase Causing turbulence Cooling and wetting of surface Halon has been phased out because of its effects of the ozone layer. It an ozone depleting substance as it causes depletion of the ozone layer (United States, 2002). Part E. (LO 1) Heat transfer The mean thermal inertia of skin has been estimated as 1.7 kW s1/2m-2K-1. Formula (3) estimates the surface temperature of skin exposed to a constant heat flux: The formula for estimating the surface temperature of the skin is Where T is 440C = 317K which is the temperature human skin begins to feel pain T0 = 310K (= 370C which is the normal body temperature) β = 1.7 kW s1/2m-2K-1 mean thermal inertia Q” = 80wm-2s-1 which is constant t =? The time for a person to feel pain when exposed to constant heat flux of 80wm-2s-1 is found from the following equation. This means that the person will feel pain after 5hrs. How would the “thermal penetration depth” of skin vary with time for someone in a developing room-fire environment? If the heat flux to the skin was a steady 100 kJ m-2 s-1 when would a person with normal pain threshold and skin texture experience pain and a burn: Table: Effects on human body when exposed to for different temperatures Part F. (LO 1) Electronics An extension lead is 2m long and carries current down a 2.5 mm2 copper conducting wire, driven by a 230V potential difference. If the lead is drawing 13 Amps current and 0.1% of the energy is lost to heating the wire, how hot would it get after an hour? Using the formula Vit = mcT V = 230V, I = 13A, t = 60 x 60 s, m =, Assuming that the initial temperature is 293K Then the temperature after an hour will be 6,154,846K References Currie I.G. (2012). Fundamental Mechanics of Fluids, Fourth Edition Civil and mechanical engineering, CRC Press Hurley, M. J. (2016). SFPE handbook of fire protection engineering. International Association of Fire Chiefs, 2004. Fundamentals of Fire Fighter Skills, Jones & Bartlett Learning Siegel, J. A., & Saukko, P. J. (2013). Encyclopedia of forensic sciences. United States. (2002). Amendments to the Montreal Protocol on substances that deplete the ozone layer: Report (to accompany Treaty docs. 106-10 and 106-32). Washington, D.C.: U.S. G.P.O. Draft nternal energy. The term III represents the influence by meanpressure gradients. The term IV represents the influence due to the conversionof mechanical energy into thermal energy as a result of the action of the stresses on the surface. The term V represents the rate at which heat isincreased due to convection from outside (Currie, 2012). Part D. (LO 4) Firesupression (a) Analyse three conditions essential for combustion and fire (thefire triangle). Specify three associated methods of fire-fighting and relatethese methods with the action of water, foam, or neutral gas. (5 marks) Thecombustion and fire occurs in the presence of three conditions which interacttogether. The conditions are: - A source of heat (thermal energy or activation energy) - An oxidizer (oxygen) - Fuel (combustible material) The conditions arenormally represented in fire triangle as shown below. Each side of the trianglerepresents one condition and the triangle symbolizes the interaction among thethree conditions, which produce fire (Siegel, & Saukko, 2013). Fire isstopped by removing one side of the triangle. The principle of fireextinguishment is to inhibit the interaction of one condition with the othertwo. The basic methods that are associated with the fire triangle include: ?Cooling the burning material ? Excluding oxygen from fire ? Removing fuel fromfire (Siegel, & Saukko, 2013) (b) Critically review different mechanisms offire extinguishment (cooling of flame, reduction of fuel and/or oxygen, andinterference with combustion reactions). (10 marks) Cooling of flame The mostcommon method for extinguishing fire is cooling the burning material usingwater. Water absorbs heat from fire as it changes into steam. It serves as aheat sink by reducing the temperature of the fuel. The gaseous fuel can also becooled by use of water vapour as it has a relatively higher heat capacitycompared to most fuel. If the fuel composed of a hydrocarbon, water will spreadthe flame as they are immiscible. Reduction of fuel and/or oxygen Another methodis application of foam to a petroleum fire. It not only cools the surface of the fuel, but it also creates a blanket over it which separates the oxygen from thefuel, and prevents any gasified fuel from rekindling the flame. Interferencewith combustion reactions The method involves blocking the fuel by closing thesupply pipes. The supply of oxygen can also be stopped by closing the openingsto the burning fuel. This technique causes depletion of oxygen at the atmospheresurrounding the flame and the flame is extinguished (International Associationof Fire Chiefs, 2004). (c) Review fire protection using water. Analyse thereason for Halon phase-out. (5 marks) The use of water to distinguish fire hasbeen common. Water mist is used to suppress fire by generating and distributingwater mist within the protected space. Small droplets of mists extinguish theflame through the combined effects of cooling and dilution of oxygen 1 1 1 through thesteam generated after evaporating water. The techniques used to generate watermists are: ? High pressure single fluid atomisation ? Dual fluid atomization ?Hot water steam generation ? Low pressure single fluid atomisation The watermists droplets used to extinguish fire has diameter of less than 200Â m. Thistechnique uses mechanisms such as: ? Steam formation resulting in oxygendilution ? Cooling of the gas phase ? Causing turbulence ? Cooling and wettingof surface Halon has been phased out because of its effects of the ozone layer.It an ozone depleting substance as it causes depletion of the ozone layer(United States, 2002). Part E. (LO 1) Heat transfer The mean thermal inertia ? ?k?c of skin has been estimated as 1.7 kW s1/2m-2K-1. Formula (3) estimates thesurface temperature of skin exposed to a constant heat flux:The formula for Draft nternal energy. The term III represents the influence by meanpressure gradients. The term IV represents the influence due to the conversionof mechanical energy into thermal energy as a result of the action of the stresses on the surface. The term V represents the rate at which heat isincreased due to convection from outside (Currie, 2012). Part D. (LO 4) Firesupression (a) Analyse three conditions essential for combustion and fire (thefire triangle). Specify three associated methods of fire-fighting and relatethese methods with the action of water, foam, or neutral gas. (5 marks) Thecombustion and fire occurs in the presence of three conditions which interacttogether. The conditions are: - A source of heat (thermal energy or activation energy) - An oxidizer (oxygen) - Fuel (combustible material) The conditions arenormally represented in fire triangle as shown below. Each side of the trianglerepresents one condition and the triangle symbolizes the interaction among thethree conditions, which produce fire (Siegel, & Saukko, 2013). Fire isstopped by removing one side of the triangle. The principle of fireextinguishment is to inhibit the interaction of one condition with the othertwo. The basic methods that are associated with the fire triangle include: ?Cooling the burning material ? Excluding oxygen from fire ? Removing fuel fromfire (Siegel, & Saukko, 2013) (b) Critically review different mechanisms offire extinguishment (cooling of flame, reduction of fuel and/or oxygen, andinterference with combustion reactions). (10 marks) Cooling of flame The mostcommon method for extinguishing fire is cooling the burning material usingwater. Water absorbs heat from fire as it changes into steam. It serves as aheat sink by reducing the temperature of the fuel. The gaseous fuel can also becooled by use of water vapour as it has a relatively higher heat capacitycompared to most fuel. If the fuel composed of a hydrocarbon, water will spreadthe flame as they are immiscible. Reduction of fuel and/or oxygen Another methodis application of foam to a petroleum fire. It not only cools the surface of the fuel, but it also creates a blanket over it which separates the oxygen from thefuel, and prevents any gasified fuel from rekindling the flame. Interferencewith combustion reactions The method involves blocking the fuel by closing thesupply pipes. The supply of oxygen can also be stopped by closing the openingsto the burning fuel. This technique causes depletion of oxygen at the atmospheresurrounding the flame and the flame is extinguished (International Associationof Fire Chiefs, 2004). (c) Review fire protection using water. Analyse thereason for Halon phase-out. (5 marks) The use of water to distinguish fire hasbeen common. Water mist is used to suppress fire by generating and distributingwater mist within the protected space. Small droplets of mists extinguish theflame through the combined effects of cooling and dilution of oxygen 1 1 1 through thesteam generated after evaporating water. The techniques used to generate watermists are: ? High pressure single fluid atomisation ? Dual fluid atomization ?Hot water steam generation ? Low pressure single fluid atomisation The watermists droplets used to extinguish fire has diameter of less than 200Â m. Thistechnique uses mechanisms such as: ? Steam formation resulting in oxygendilution ? Cooling of the gas phase ? Causing turbulence ? Cooling and wettingof surface Halon has been phased out because of its effects of the ozone layer.It an ozone depleting substance as it causes depletion of the ozone layer(United States, 2002). Part E. (LO 1) Heat transfer The mean thermal inertia ? ?k?c of skin has been estimated as 1.7 kW s1/2m-2K-1. Formula (3) estimates thesurface temperature of skin exposed to a constant heat flux:The formula for Read More